: The overall objective of this proposal is to explore the mechanisms for antifibrotic role of nitric oxide on myocardial remodeling following infarction. The hypothesis to be tested is that nitric oxide regulates myofibroblasts (myoFb) apoptosis/growth, collagen turnover, and transforming growth factor (TGF)-beta1 expression in the infarcted heart, thus protecting the heart from ongoing collagen accumulation. Extensive myocardial fibrosis is a characteristic feature in the failing heart with previous myocardial infarction (MI). Locally generated factors serve as chemical mediators by either promoting or suppressing cardiac fibrosis. A discordant balance in reciprocal regulation accounts for progressive structural remodeling of the myocardium. An emerging body of evidence implicates nitric oxide as an inhibitory regulator of cardiac fibrosis. The pathway by which nitric oxide regulates such cardiac remodeling remains uncertain, MyoFb, phenotypically transformed fibroblast-like cells, are central for cardiac extracellular matrix remodeling and create a dynamic microenvironment for collagen turnover in the infarcted heart. By using a mouse model of MI and inducible nitric oxide synthase gene knockout mice together with cultured myoFb obtained from infarcted heart, the following specific aims will be fulfilled. Aim 1: to determine whether nitric oxide regulates myoFb growth/survival and activity in the infarcted heart by characterizing its influence on myoFb apoptosis and regulatory genes, myoFb replication and phenotype change. Aim 2: to detect whether nitric oxide is involved in collagen turnover by detecting its potential role in metalloproteinase-1 (MMP-1) synthesis, MMP-1 activation, tissue-derived MMP inhibitors expression, and cardiac collagen synthesis. Aim 3: to determine whether nitric oxide modulated TGF-beta1 expression and activity at sites of cardiac fibrosis by investigating its effect on TGF-beta1 synthesis, activation and receptor expression. Findings derived from the proposed studies should advance our understanding on pathogenesis and management cardiac remodeling by fibrous tissue, which will provide the opportunity for pharmacologic interventions that limit collagen deposition in the heart with MI and improve cardiac function.